1. Field of the Invention
The present invention pertains to a treating agent for forming a protective coating, metallic materials that have a protective coating, and a method for forming a protective coating. In more detail, the present invention pertains to a treating agent for forming a protective coating with excellent workability and galling resistance, also heat resistance to working heat, and a rust-preventive property on the surfaces of metallic materials that require cold plastic working; to metallic materials that have a protective coating; and to a method for forming a protective coating, and can be used by applying a lubricating component on the surface of the coating or by incorporating it in the coating, thereby providing the lubricity necessary for working.
2. Discussion of the Related Art
In the cold plastic working of metallic materials as typified by forging, wire drawing, and pipe extraction and drawing, a protective coating layer is formed on the surface of a workpiece for the purpose of preventing galling between the workpiece and the tool being used. The primary function of the protective coating layer is to prevent direct metallic contact between the workpiece surface and tool surface; strong adhesion of the protective coating to the surface of the metallic material and the ability of the protective coating to follow expansion of the surface area during plastic working are considered to be particularly important. Furthermore, a lubricating component is applied on the protective coating layer or incorporated within it, so as to reduce the coefficient of friction with the tool surface and to ease the load on the coating layer itself and the generation of working heat, thus actually reducing the working energy.
Conventional techniques for forming such a protective coating layer include various techniques that have been investigated many times in the past. The methods that have typically been used include the direct formation of a coating film consisting mainly of a lubricating component, such as an oil, soap, metallic soap, or wax film, and the formation of a reactive chemical conversion coating layer, such as a phosphate or oxalate film, on the surface of a metal, and finally the formation of a lubricating component film. The former has advantages such as fewer treatment steps and simple and easy control of the solution, because the coating can be obtained by applying the lubricating component singly or, if needed, together with a binder component on the surface of a workpiece, followed by drying. However, these coatings do not adhere sufficiently to the workpiece surface, and more importantly, cannot follow the expansion of the surface area of the material in areas that are intensely worked, which means that extremely thinned-out film areas or broken film areas develop, and accordingly, in many cases, the resulting protective film function is insufficient and/or cannot be maintained.
In the latter method, on the other hand, a compact reactive chemical conversion coating layer is formed on the workpiece surface, thereby providing strong adhesion to the material surface, and also exhibiting a sufficient “following” property with respect to the expansion of the surface area during intense working. Furthermore, the adhesion and retention of a metallic soap layer that is formed by a reaction with the surface layer of the chemical conversion coating layer, and a sodium soap layer as the lubricating component of the topmost layer are secured, the latter through the surface roughness, so that work energy is actually reduced considerably. However, because the chemical conversion coating is formed by a chemical reaction, complicated treatment solution control and many steps are required, and in addition, the costs required are extremely high if wastewater treatment and facility investment are included. Moreover, a heavy load is placed on the global environment, because industrial waste, such as chemically produced sludge, is produced in large quantities. In addition, the chemical reactivity differs considerably, depending on the object material, hence the latter method has another drawback in that it is difficult to apply to difficult-to-convert materials that exhibit poor chemical reactivity.
To solve these problems, effort has been expended to improve the performance of protective coatings by the former method to a degree that is comparable to that of a chemical conversion treatment. As a result of these efforts, methods using oil-based lubricating agents or water-based lubricating agents have been proposed. Japanese Patent Kokoku No. 4[1992]-1,798 discloses the following oil-based lubricating agents: lubricating agents for cold working prepared by adding metallic soap or solid lubricants to lubricating oils, which are prepared by mixing extreme-pressure agents such as chlorinated paraffins or phosphates, copolymers of isobutylene and n-butene, and animal and plant oils. However, even these high-performance lubricating agents present some problems related to workability when compared with a lubricating process where reactive soap lubrication is applied after chemical conversion film formation, and thus there can be drawbacks such as the generation of foul odors during working and the contamination of oil systems, because of the use of extreme-pressure additives, while a cleaner environments is called for in workplaces such as the sites at which the actual working is carried out.
Recent techniques for aqueous lubricants that have been disclosed include lubricant compositions for the plastic working of metallic materials containing synthetic resins and water-soluble inorganic salts in specified ratios (Japanese Patent Kokai 2000-63,880). These techniques are used for the purpose of avoiding direct metallic contact with a tool by forming a coating film with a uniformly precipitated synthetic resin and water-soluble inorganic salt on the surface of a workpiece, followed by the incorporation of a lubricating component in the coating in any desired ratio, and are thus supposed to provide performance properties that are comparable or superior to those of a phosphate coating on which a lubricating component layer is formed.
However, by and for themselves, these coatings bear two functions, namely, an anti-galling property and a lubricating property, and in practical terms are often applied for a barrier effect to protect the material surface from corrosion before and after working, but they are actually extremely poor in rust-preventive property when compared with a combination of a phosphate coating and a lubricating component layer, mainly because the amount of coating remaining on the material is sharply reduced after working.